Dynamic Assemblies

using System;
using System.Collections.Generic;
using System.Text;
using System.Reflection;
using System.Reflection.Emit;
using System.Threading;

namespace DynamicAssemblyDemoWithEmit
{
    class DynamicAssemblyCreator
    {
        object bookShopRulesObj;

        public DynamicAssemblyCreator()
        {
            Assembly a = this.CreatAssembly();
            bookShopRulesObj = a.CreateInstance("BookShopRuleWithEmit.BookShopRules");
        }

        public double GetDiscount(double price , double qty)
        {
            DynamicAssemblyCreator t = new DynamicAssemblyCreator();
            Assembly a = t.CreatAssembly();

            try
            {
                if (bookShopRulesObj != null)
                {
                    bookShopRulesObj.GetType().GetField("price").SetValue(bookShopRulesObj, price);
                    bookShopRulesObj.GetType().GetField("quantity").SetValue(bookShopRulesObj, qty);

                    MethodInfo methodInfo = bookShopRulesObj.GetType().GetMethod("GetDiscount");
                    Object commission = methodInfo.Invoke(bookShopRulesObj, null);
                    if (commission is double)
                    {
                        return (Convert.ToDouble( commission) );
                    }
                }
                else
                {
                    Console.WriteLine("Error!");
                }
            }
            catch (Exception e)
            {
                Console.WriteLine("Error!   " + e.Message);
            }

            return 0;

        }

        /// <summary>
        /// Creating the emit dynamic dll
        /// </summary>
        /// <returns></returns>
        public Assembly CreatAssembly()
        {
            AssemblyBuilder ab = null;
            try
            {
                AssemblyName an = new AssemblyName(); // we can define asseblies unique identity
                // Version is 1.0.0.0
                an.Version = new Version(1, 0, 0, 0);
                // Set the assembly name
                an.Name = "BookShopRuleWithEmit";

                // Define a dynamic assembly
                // AssemblyBuilderAccess : Defines the access mode of the assembly use Run to avoid assembly creation.
                ab = Thread.GetDomain().DefineDynamicAssembly(an, AssemblyBuilderAccess.RunAndSave);

                // Define a dynamic module and the filename of the assembly
                ModuleBuilder modBuilder = ab.DefineDynamicModule("BookShopRuleWithEmit", "BookShopRuleWithEmit.dll");

                // Create the public BookShopRules class (can spesify the namespace if needed)
                TypeBuilder tb = modBuilder.DefineType("BookShopRuleWithEmit.BookShopRules", TypeAttributes.Public);

                // Define two fields (both Public)
                FieldBuilder price = tb.DefineField("price", typeof(double), FieldAttributes.Public);
                FieldBuilder qty = tb.DefineField("quantity", typeof(double), FieldAttributes.Public);


                // Define GetDiscount public method, with retuen type double
                MethodBuilder adderBldr = tb.DefineMethod("GetDiscount",
                               MethodAttributes.Public, CallingConventions.Standard,
                               typeof(double),
                               new Type[0]);
                //now the implementaion of the method
                ILGenerator ilgen = adderBldr.GetILGenerator();


                Label failed = ilgen.DefineLabel();
                Label failed2 = ilgen.DefineLabel();
                Label endOfMthd = ilgen.DefineLabel();


                ilgen.Emit(OpCodes.Ldarg_0);
                ilgen.Emit(OpCodes.Ldfld, qty);
                ilgen.Emit(OpCodes.Ldarg_0);         //condition one if((price*quantity) < 100)                                   
                ilgen.Emit(OpCodes.Ldfld, price);                       //return ((price*quantity)*.025)
                ilgen.Emit(OpCodes.Mul);
                ilgen.Emit(OpCodes.Ldc_R8, 100.00);
                ilgen.Emit(OpCodes.Bgt_S, failed);

               
                ilgen.Emit(OpCodes.Ldarg_0);
                ilgen.Emit(OpCodes.Ldfld, qty);
                ilgen.Emit(OpCodes.Ldarg_0);
                ilgen.Emit(OpCodes.Ldfld, price);       // Body of the statement
                ilgen.Emit(OpCodes.Mul);
                ilgen.Emit(OpCodes.Ldc_R8, 0.025);
                ilgen.Emit(OpCodes.Mul);
                ilgen.Emit(OpCodes.Br_S, endOfMthd);



                ilgen.MarkLabel(failed);
                ilgen.Emit(OpCodes.Ldarg_0);            // condition two
                ilgen.Emit(OpCodes.Ldfld, qty);         // else if ((price*quantity) < 500)  
                ilgen.Emit(OpCodes.Ldarg_0);                        //return ((price*quantity)*.05)
                ilgen.Emit(OpCodes.Ldfld, price);
                ilgen.Emit(OpCodes.Mul);
                ilgen.Emit(OpCodes.Ldc_R8, 500.00);
                ilgen.Emit(OpCodes.Bgt_S, failed2);

                ilgen.Emit(OpCodes.Ldarg_0);
                ilgen.Emit(OpCodes.Ldfld, qty);
                ilgen.Emit(OpCodes.Ldarg_0);
                ilgen.Emit(OpCodes.Ldfld, price);          // body of else if
                ilgen.Emit(OpCodes.Mul);
                ilgen.Emit(OpCodes.Ldc_R8, 0.05);
                ilgen.Emit(OpCodes.Mul);
                ilgen.Emit(OpCodes.Br_S, endOfMthd);


                ilgen.MarkLabel(failed2);
                ilgen.Emit(OpCodes.Ldarg_0);
                ilgen.Emit(OpCodes.Ldfld, qty);
                ilgen.Emit(OpCodes.Ldarg_0);
                ilgen.Emit(OpCodes.Ldfld, price);          // body of else return  ((price*quantity)*.075)
                ilgen.Emit(OpCodes.Mul);
                ilgen.Emit(OpCodes.Ldc_R8, 0.075);
                ilgen.Emit(OpCodes.Mul);
                ilgen.Emit(OpCodes.Br_S, endOfMthd);

                ilgen.MarkLabel(endOfMthd);                 // returning from the method
                ilgen.Emit(OpCodes.Ret);


                tb.CreateType();
                ab.Save("BookShopRuleWithEmit.dll");

            }
            catch (Exception e)
            {

            }

            return ab;
        }

    }
}